Joining of advanced materials by superplastic deformation

ABSTRACT

A method for utilizing superplastic deformation with or without a novel joint compound that leads to the joining of advanced ceramic materials, intermetallics, and cermets. A joint formed by this approach is as strong as or stronger than the materials joined. The method does not require elaborate surface preparation or application techniques.

This Application is a Divisional of U.S. patent application Ser. No.09/924,571, filed on Aug. 7, 2001, now U.S. Pat. No. 6,974,070.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. W-31-109-ENG-38 between the U.S. Department of Energy and theUniversity of Chicago representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for joining togethermultiphase objects, and more particularly, the present invention relatesto methods for joining together ceramic shapes to form pore-free jointsor junctions at least as strong as the materials being joined, and tojoin ceramic/metal composites (cermets).

2. Background of the Invention

Ceramics in general are difficult to form into complex shapes. Atpresent, complex ceramic shapes are often prepared by forming thecomplex shape in the green state and then applying heat to consolidatethe shape. This can be difficult and costly.

An alternative method is to form a ceramic blank and then machine theblank into the desired shape. This method also can be difficult, timeconsuming, expensive, and can introduce faults in the structure, whichwill reduce performance of the structure significantly or possiblyrender the shape unusable.

Another alternative is to join simpler shapes to form the desiredcomplex shapes. This alternative is similar to brazing or welding ofvarious metal components to form more complex metal components. Ceramiccomponents have been joined using various glasses and metals as thejoining material. However, the resulting joints have poor mechanicalproperties compared to the materials to be joined, and the applicationtemperatures are limited.

Nanocrystalline materials have been used as joint-forming interlayerconstituents between the two ceramic shapes to be joined. F.Gutierrez-Mora, A Dominguez-Rodriguez, J. L. Routbort, R. Chaim, and F.Guiberteau, “Joining of yttria-tetragonal stabilized zirconiapolycrystals (Y-TZP) using nanocrystals,” Scripta Mater., 41, 455-460(1999). However, nanocrystalline materials can be very expensive and canbe difficult to work with. Nanocrystalline ceramics are difficult toconsolidate into a dense body without having the individual grains growsubstantially. Growth of the grains eliminates the nanocrystallinenature of the body and renders another common ceramic.

Nanocrystalline materials have other drawbacks. For example, suchpowders tend to agglomerate badly, hence making it difficult to producedense bodies or to apply materials uniformly to a surface. Also,nanocrystalline powders are often highly hygroscopic. Adsorption ofmoisture can make it difficult or impossible to process nanocrystallineceramic powders into strong, pore-free bodies.

A process called superplastic deformation has been used to join Y-TZP ofthe same composition as that mentioned in the F. Gutierrez-Morareference, supra.; J Ye. et al., Scripta Metall. Mater., 33, pp 441-445(1995); and A. Dominguez-Rodriquez, et al, J. Mater. Res., 39,p1631-1636 (1998). Superplasticity in deformation of materials usuallyoccurs by a process known as grain-boundary sliding. At elevatedtemperature, under application of a stress, individual grains of thesolid slide and rotate past each other so that permanent deformation cantake place. Generally for superplasticity to occur, individual grainsmust remain virtually stable. They cannot grow or change shapesignificantly, nor should they react with other species present. Ifgrain growth occurs, superplasticity is prevented. As grains grow duringdeformation, small pores are created. These pores grow and eventuallyjoin to form cracks which reduce the ceramic strength.

Many previous efforts using nanocrystalline materials to join objectshave resulted in a joint containing inferior, stable residual porosity.R. Chaim et al., J. Mater. Res., 15, pp 1724-1728 (2000). Thus, in theseand other similar instances, an expensive, difficult-to-prepareinterlayer was required. Either that, or settle for an inferior joint.

Aside from joining ceramic forms, it is also desirous to join shapescomprised of cermets. Cermets are ceramic/metal composites in whichceramic particles are the majority phase by volume. Most cermets containbetween 5 and 15 volume percent metal to bind hard ceramic particlessuch as WC and TiC. Cermets are employed in applications such as cuttingtools in which wear resistance is required.

With respect to joining simple cermets to form more complex structures,such as, for example, serrated cutting edges, no joining technology hasbeen found to be widely successful. Conventional welding is ineffectivebecause, at welding temperatures, the ceramic is solid but the metallicphase is liquid. Leaching of the metal and destruction of the cermetoccurs during welding.

Conventional brazing or soldering forms joints with insufficientstrength for many applications. Also, the resulting joints have poorresistance to heat. Because cutting and grinding operations oftenproduce substantial heating of the cermet tooling, and the stresses onthe tooling are high, brazed or soldered joints will fail in mostapplications.

Complex cermet tooling is typically fabricated to shape. The requiredprocedures make use of intricate and relatively expensive dies ordiamond-tooling machining. Furthermore, because of geometric constraintsassociated with part removal from a die, the shapes that can be formedare limited.

U.S. Pat. No. 6,168,071 awarded to Johns on Jan. 2, 2001 discloses amethod for joining materials together by a diffusion process usingsilver/germanium alloys. No external pressure is applied.

U.S. Pat. No. 5,855,313 awarded to McAfee et al. on Jan. 5, 1999discloses a method for a two-step brazing process for joining materialswith different coefficients of thermal expansion.

U.S. Pat. No. 5,599,419 awarded to Hunter et al. on Feb. 4, 1997discloses a method for joining plastic materials via a heated bladewhich simultaneously heats the two surfaces to be joined. The heatedblade is removed and the two surfaces are welded together.

U.S. Pat. No. 4,927,475 awarded to Steinleitner et al. on May 22,1990discloses a method for joining surfaces of different materials byapplying a glass coating to the two surfaces to be joined. The surfacesare subsequently joined while heated and under external pressure.

U.S. Pat. No. 4,414,166 awarded to Charlson et al. on Nov. 8, 1983discloses a method for laser joining of thermoplastic and thermosettingmaterials by laser radiant energy which causes the thermoplasticmaterial to flow onto the thermosetting material. No external pressureis applied.

U.S. Pat. No. 4,247,345 awarded to Kadija et al. on Jan. 27, 1981discloses a method for joining sections of synthetic materials byplacing a thermoplastic sealing composition in a gap between thematerials to be joined and then binding the materials together withsubsequent heating of the sealing composition. No external pressure isapplied.

A need exists in the art for a ceramic joint forming process thatresults in joints as strong, or actually stronger than the materialsjoined. The process should be simple in that no elaborate surfacepreparation or application techniques are required. The process alsoshould utilize common ceramic materials and readily available equipmentto minimize costs. Joining temperatures should be as low as possible tominimize degradation of the ceramics being joined and to minimize thecost and complexity of the tooling needed to form the joints.

There is also a need for a technology for producing robust complexcermet forms from simpler ones. The technology should be simple,inexpensive, require few steps to complete, and require minimal surfacepreparation. Cermets are difficult to polish (inasmuch as theythemselves are used to polish other materials), and so polishing of thecermets resulting from the technology should not be required.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for joiningmaterials that overcomes many of the disadvantages of the prior art.

Another object of the present invention is to provide methods forjoining materials wherein the joint formed will actually be strongerthan the multiphase materials it joins. A feature of the invented methodis the relative stability of the mixtures utilized. An advantage of theinvented method is that the propensity for grain growth is greatlyreduced through the use of relatively low temperatures.

Yet another object of the present invention is to provide methods forjoining multiphase materials that do not require elaborate surfacepreparation or application techniques. A feature of the invented methodfor joining cermets is the use of a joint compound that contains a metalalso found in the cermet. An advantage of the invented method is thatthe equipment needed is relatively inexpensive and readily available.

Still another object of the present invention is to provide a method forseamlessly joining ceramic objects to form a construct. A feature of theinvented method is that the grain size of the joint compound is smallerthan the grain sizes of the constituents making up the shapes joined. Anadvantage of the invented method is that the strength of the joint isfurther enhanced due to the lack of porosity which can otherwise weakenthe joint.

Another object of the present invention is to provide a method forjoining similar or dissimilar multiphase objects. A feature of theinvented method is that optimal conditions of temperature and pressureallow for joint formation without an interlayer (i.e. joint compound).An advantage of the invented method is its simplicity of design andoperation.

In brief, the invention provides a joint compound to seamlessly joinmultiphase objects, the compound comprising a first phase, and a secondphase mixed with said first phase to create a mixture, wherein saidsecond phase is kinetically stable to said first phase.

The invention also provides a method for seamlessly joining objects madeup of certain sized particles, the method comprising: supplying a jointcompound having particle sizes smaller than the certain sized particles;applying the joining compound to opposing surfaces of the objects to bejoined together; heating the joint to a temperature below the meltingpoint of the lowest melting point constituent of the construct; andapplying pressure to the objects so as to direct the surfaces towardeach other to create a construct, whereby the joint compound isintermediate the opposing surfaces.

Also provided is a method for seamlessly joining together objects madeof cermet, the method comprising selecting opposing surfaces of theobjects having surface finishes as defined by root-mean-square values ofless than 50 microns; coating the surfaces with a metal fluid;decomposing the metal fluid so as to leave a metal residue on thesurfaces; and contacting the surfaces to each other for a time and at atemperature and pressure sufficient to form an irreversible bond betweenthe objects.

The invention provides a construct comprising a first crystalline solidhaving a first surface directly bonded to a second surface of a secondcrystalline solid, wherein the finish of the first surface and secondsurface are less than or equal to two microns (equivalent to adiamond-saw cut surface), as defined by standard root-mean-squarevalues.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome readily apparent upon consideration of the following detaileddescription and attached drawing, wherein:

FIG. 1 is a schematic depiction of the invented plastic deformationjoining process, in accordance with features of the present invention;

FIG. 2 is a Scanning Electron Microscope (SEM) photomicrograph of ajoint between ZT80A (80% Al₂O₃/20% ZrO₂) and ZT20A (20% Al₂O₃/80% ZrO₂),in accordance with features of the present invention;

FIG. 3 is a depiction of the residual stresses in a joint formed byjoining ZT20A and ZT80A with a joint compound which is ZT50A (50%Al₂O₃/50% ZrO₂);

FIG. 4 is a graph depicting residual stresses measured by indentationtechniques, in accordance with features of the present invention;

FIG. 5 is a photomicrograph of strengthening agents dispersed in aformulated joint, in accordance with features of the present invention;and

FIG. 6 is a photomicrograph of a cobalt interlayer used to join cermetshapes, in accordance with features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Low-cost, highly optimized methods for joining difficult-to-joinmaterials via plastic deformation are provided. The invented processallows the joining of materials of different compositions, withresulting joints that are seamless (i.e., without pores, cracks, orother discontinuities). These joints are as strong or stronger than thestructures they bond. By selecting the appropriate ratios of phases inthe joint, because of differences in thermal expansion coefficientsbetween constituents, the inventors have been able to tailor residualstresses so that the overall strength of the resulting construct ismaximized.

Salient features of the invented warm-to-hot pressing method include theuse of joint forming compounds which do not employ nanocrystallinematerials. In some instances, no joint-forming compound whatsoever isrequired. In cermets, where joint compound is used, in the most commonapplication of the invention, the joint has a composition containing thehost metal (i.e., a metal also contained in the cermet structures to bejoined). Other scenarios utilize joint compound having constituents notsimilar to those comprising the objects to be joined.

The invention provides for joining shapes by a process that comprisesheating the components to an elevated temperature; pressing the shapestogether; and doing so with or without use of a joint compound betweenthe shapes to be joined. FIG. 1 depicts the joining process, generallydesignated as numeral 10. A joint compound 12 is applied to opposingsurfaces 14, 16 of adjacent ceramic structures 18, 20.

Thickness δ of the joint compound is a multiple of the dimension of thelargest grain size of the components of the compound. Generally, thethickness should be above five times the thickness of the dimension ofthe largest grain size. Thicknesses of between 5 and 500 times thethickness of the dimension of the largest grain size are mostpreferable.

Optionally, to form a defect-free joint, pressure is applied to thejoint at a level sufficient to effect plastic deformation bygrain-boundary sliding. Pressures from 1,000 pounds per square inch(psi) to 45,000 psi are suitable. Typically, the pressure, designated asσ in FIG. 1, is applied so that the opposing faces 14, 16 of the bulkconstituents to be joined are directed toward each other during pressureapplication. Alternatively, pressure may be applied hydrostatically, inwhich case the entire construct is placed in a chamber and pressure isapplied by a controlled atmosphere. The atmosphere is selected toprevent deleterious reaction with the ceramic bodies. For example, ifSi₃N₄/SiC composites are to be joined, the atmosphere should be rich innitrogen and very low in oxygen. If electronic ceramic composites, suchas those based on BaCeO₃, are to be joined, the atmosphere must have anoxygen partial pressure of at least 1 part per million.

Multiphase compounds are joined by the invented method. Thisincorporation of multiphase materials eliminates the need of expensivenanocrystalline materials. This is because the joint material itselfcontains sufficient superplastic material to allow for formation ofperfect joints. The multiphase materials also permit the formulation offunctionally graded materials that exhibit spatially controlledproperties, compositions, and residual-stress states.

In multiphase materials, each component has a characteristic thermalexpansion coefficient that is different from those of the otherconstituents. With cooling from the joining temperature, residualstresses will develop in the phases of any material. By selecting whichmaterials are present where, and in what concentrations, the inventorshave succeeded in tailoring residual stresses to specific sites of theresulting construct.

For illustrative purposes herein, the multiphase compounds (e.g.,two-phase compounds) utilized in the invented protocols are one of twotypes: (1) ceramics or other materials, in which the crystals of allphases are hard (hardness greater than 1150 on the Knoop scale) andnearly undeformable at room temperature; (2) cermets, in which thecrystals of the majority phase are hard and undeformable and theminority, matrix phase is a metal that is deformable at the temperatureand stress levels used to form the joint.

Constructs resulting from the joining process can be either homogeneousin constituency (wherein for example, electronic ceramics are joined toelectronic ceramics), or heterogeneous in constituency (wherein, forexample, electronic ceramics are joined to cermets, or intermetallics,or both). Each of these classes of materials can be joined with orwithout a joint material between the pieces to be joined. The onlyrequirement is that the objects that are combined into a single piece becomposed of stable microstructures (i.e. crystalline grains) that candeform by grain-boundary sliding. In this context, stability refers tolack of significant grain growth of reaction during processing.

In light of the foregoing, while the bulk of this specification detailsthe joining of ceramic and cermet materials, the invented process can beutilized to join a myriad of materials, including but not limited toceramics, glass-ceramics, hard intermetallics, metals, and combinationsthereof.

Residual Stress Detail

The inventors have found that they can place tensile and compressivestresses in specific joint regions where they will (a) do the most goodby providing residual stresses that increase the joint strength, (b) dothe least harm, or (c) do both. For example, selection of theappropriate ratio of phases in the joint allows one to tailor residualstresses so that the overall strength of the resulting construct ismaximized. Stresses can be tailored as to location, magnitude anddirection in any stable two- or otherwise multiple-phase compositesystem.

As can be noted in FIG. 3, which is discussed in more detail infra,joints which result from the invented process cause a more gradualdistribution of stress across subunits of the fabricated construct.Residual stresses can be beneficial, as in the case of compressivestresses acting to minimize crack propagation, or deleterious, as in thecase of large tensile stresses promoting crack propagation.

Constituent Ratio and Type for Ceramics Joining Detail

In joining of ceramics and other composite materials in which theindividual crystals are brittle and essentially undeformable, theinventors have confirmed that there are specific requirements for thevarious constituents:

First, phases that come in contact must be kinetically stable withrespect to each other. Several ceramic systems exhibit such stability.Examples include, but are not limited to, Al₂O₃ and ZrO₂; Al₂O₃ and YSZ;Al₂O₃ and SiC; Al₂O₃ and Y₂O₃; Al₂O₃ and mullite; mullite and SiO₂;ZrSiO₄ and SiO₂; Al₂O₃ and Y₃Al₅O₁₃; Si₃N₄ and SiC. In addition, morethan two phases can be present. For example, the three-phase system ofAl₂O₃, YSZ, and MgAl₂O₄ is kinetically stable. During the bondingprocess, no gross reaction can occur between any of the phases.

Second, at least one of the constituents of the structure (wherein thestructure comprises the bulk material and optionally, a joint compoundmaterial) should consist of 65% or more by volume of a phase thatexhibits superplastic flow at the temperature at which the joint isformed. Superplastic deformation in ceramics can allow for stretching intension to strains >100%. That is, the ceramic being stretched candouble its initial length. Values in excess of 10-times the initiallength have been noted.

The inventors have determined that most preferable ceramics and otherhard materials that deform superplastically consist of microstructuresequiaxed grains (i.e., the grain aspect ratio is approximately 1. )These materials will deform superplastically if the temperature is keptsufficiently low to prevent significant grain growth (inasmuch asdeformation rate is a very strong function of temperature).Alternatively, the microstructure must be such that its propensity forgrain growth is inherently small. The invention makes use of this typeof inherently stable microstructure.

Generally, ceramics and other hard materials to be joined must consistof a mixture of kinetically stable phases in which no one phase makes upmore than 85% by volume of the total. This constraint allows for pinningof grain boundaries. The 15% or more of added phases acts to preventgrowth of the surrounding grains. In effect, the grains act as mutualobstacles to each other's growth.

The invention does not require, in contrast to previous technologies,that any of the individual components (i.e., either the joined pieces orthe joint compound) be entirely superplastic at the temperatures andpressures used for joining. It requires only that approximately 65% ormore any one of the components consist of a superplastic material. Theother 35% (or less) may consist of any chemically compatible species.This option allows for incorporation of, for example, toughening agentsor for functional materials of various forms (for example, addition ofan electronically conducting material such as TiC to the insulatorAl₂O₃). Exemplary toughening agent species include, but are not limitedto platelet powders, whiskers and chopped fibers, said entitlescomprised of Al₂O₃, mullite, Si₃N₄, SiC, TiC, and Y₃Al₅O₁₃; SiC/Si₃N₄,Al₂O₃/mullite, or combinations thereof. Unreactive metals also can beutilized, including, but not limited to, W, Pt, Pd, Au—Pd alloys, Ag—Pdalloys, Ni, or Co.

For ceramics, the various phases must be kinetically stable with respectto each other. That is, at the temperature at which the joint is formed,the individual phases must remain distinct and intact. No significantreaction can occur during the joining. The phases may bethermodynamically stable with respect to each other; that is, noreaction occurs at the temperature of joining, irrespective of time.Alternatively, the phases may be reactive, but the reaction must besufficiently slow so that no significant reactions occur during joining.In the invention, maximum times at the highest temperature are less thanthree (<3) hours. The phases must remain distinct and intact for atleast three (3) hours.

Temperature Detail

The invented protocols utilize low temperatures, i.e., temperatures 600°C., and 1450° C. for ceramics and other hard materials and between 400°C. and 1100° C. for cermets. Generally, temperatures T are less than orequal to approximately 0.7 T_(m), wherein T_(m) is the melt temperatureof the lowest melting component in the resulting construct. Preferably,the temperature is selected from between 0.5 and 0.7 T_(m). The lowtemperature protocol minimizes grain growth within any joints formed,and reduces the cost of heating.

Ceramic Joint Compound Detail

Regarding the use of the invented joint compound, selection of theappropriate ratio of phases in the joint allows one to tailor residualstresses so that the overall strength of the resulting component ismaximized.

Typically, the joint compound is a suspended powder. Compoundapplication can be effected in a myriad of ways, including, but notlimited to aerosol spraying, other spray coating methods, dip-coating,screen-printing, thick-film deposition, tape-casting, doctor-blading, orelectrophoretic deposition.

Not just structural ceramics, but items made from electronic ceramics,metals, intermetallics, cermets and various other composites also can befabricated via the invented joint compound. The only requirement is thatthe objects combined be the embodiments of stable microstructures.

The compound components are kinetically stable to each other and to thematerial of the ceramic shapes to be joined. As to the grain sizes ofthe various constituents, there is a requirement that they aresufficiently small to support adequate plastic deformation. Thus, grainsizes cannot be excessively large. For the pieces to be joined, at least50 vol. % of each piece must consist of grains that are 10 microns orsmaller. The remaining 50 vol. % can have grains as large as 100microns. For the joint compound, at least 65 vol. % of the joint mustconsist of grains that are no larger than 10 microns. The remaining 35vol. % can consist of grains as large as 100 microns.

To improve mechanical properties such as fracture toughness, whiskers,platelets, chopped fibers, or stable metallic powders can be added. Forboth the pieces to be joined and the joint compound, up to 35 vol. %whiskers, fibers, platelet grains, or metals can be present. Thewhiskers or chopped fibers can be as long as 500 microns. If whiskers orfibers are present, however, at least 65 vol. % of the remaining pieceor joint material must consist of grains that are no more than 5 micronsin average size. And, these smaller grains must have aspect ratios lessthan 2, that is, they must be nearly equiaxed.

FIG. 5 shows successful addition of a ceramic whisker to a joint.

Table 1 below shows how joint toughness is increased when plateletparticles or whiskers were incorporated in the joint compound mixture.

TABLE 1 Increase in toughness to 80 vol. % Al₂O₃/20 vol. % YSZ withaddition of toughening agents Toughening agent Vol. % Percentageincrease in toughness TiC whisker 15 40 SiC whisker 15 45 SiC whisker 2560 Al₂O₃ platelet 20 10

In the invented technology, the joint material can be eliminated if thetwo pieces to be joined have the appropriate microstructures, or thejoint can be made dense and flaw-free from joint compounds that consistof inexpensive, micron-sized powders suspended in a vehicle. In thepreferred embodiment, the joint material is applied by a method that isreadily used in industry, typically aerosol-spraying or dip-coating.

Table 2 shows a representative composition for applying the jointcompound by aerosol spraying. This material is applied in 2-20 passes bya spray gun, in which a pressurized gas, such as air or nitrogen, isused to propel ceramic-containing droplets to the surfaces to be joined.

TABLE 2 Typical aerosol-spraying formulation for joining Al₂O₃/YSZceramics. Mass (g) Function Constituent 20 Powder 50 vol. % Al₂O₃/50vol. % YSZ or ZrO₂ 50 Solvent 78 wt. % xylene/22 wt. % butanol 12 BinderRohm & Haas AT-51 (Philadelphia, PA) 1.5 Plasticizer Monsanto S-160(Fayetteville, NC) 0.6 Dispersant Solsperse S-9000 (Avecia, Manchester,UK)

The resulting sprayed coating is porous and contains organic materials.The organics are removed by heating in an atmosphere that is inert orcontains oxygen. The temperature range for organics removal is 150-800°C. The time at maximum temperature is 0.1 to 5 hours. After heating, thejoint is assembled, and then the entire construct is then heated to atemperature that is below the melting temperature of the lowest meltingtemperature component in the construct. Excellent results are obtainedwhen the construct is heated to approximately 50-60 percent of themelting temperature of the lowest melting temperature component.Pore-free, i.e., 100 percent dense joints, are obtained, as depicted inFIG. 2.

The above protocol induces formation of a perfect joint. This result issurprising and unexpected. Prior to the invention, it had not proved tobe possible to form pore-free joints from what is essentially a loosepile of powder in the joint. Selection of appropriate ceramic powders(or other hard materials) so that grains do not grow, and for whichsuperplasticity is possible in at least one of the majority phases,proved to be essential for this result.

No preparation of the opposing ceramic surfaces is necessary. Thesurfaces can be either rough or smooth, or a combination thereof,without effecting the quality of the resulting joint. Preferably, themating surfaces are nearly flat.

FIG. 3 depicts the stress profile (plotted on the y-axis) vs. positionin a joint (plotted on the x-axis) in which an intermediate layer hasbeen applied. These experimental data reflect joining of a particulatecomposite of 80 vol. % Al₂O₃/20 vol. % YSZ to 20 vol. % Al₂O₃/80 vol. %YSZ. The intermediate layer is 50 vol. % Al₂O₃/50 vol. % YSZ. Theresidual hydrostatic stresses at various points are plotted as afunction of position. At the far left, indicated by a filled circle isthe residual stress present in the 80 vol. % Al₂O₃/20 vol. % YSZ ceramicpart, i.e., in the absence of a joint. On the far right, indicated by afilled triangle, is the stress present in the 20 vol. % Al₂O₃/80 vol. %YSZ ceramic part. The experimental data points, marked by X indicatethat the joint has intermediate residual stresses. That is, in theregion of the joint, the stresses are lower in magnitude that they arein the free bodies.

Joints which result from a process utilizing joint compound depict agradual shift of stresses between the subunits. As depicted in FIG. 3,hydrostatic stresses of subunits containing 80 volume percent aluminaare one-third those stresses exhibited in subunits containing 20 volumepercent alumina. However, the joint between the subunits allows forgradual transfer of said stresses between the subunits.

Surprisingly and unexpectedly, the inventors found that the jointmaterial did not have to be a superplastic material in order for aflaw-free joint to be formed. Perfect joints could be formed, in whichthe joint material consisted of whisker-, or chopped-fiber-, orplatelet-reinforced ceramic composites. The inventors determined that itwas sufficient that either the parts to be joined be superplasticmaterials, such as various Al₂O₃/YSZ mixtures, or that the joint consistof at least 65 vol. % of superplastic material. It is well known thatwhisker-containing ceramic composites do not themselves exhibitsuperplasticity. Nevertheless, the invention technology was capable offabricating flaw-free joints with intermediate layers that contained atleast 20 vol. % SiC whiskers. These whiskers are important to the finalproperties of the joint. With their addition, it is possible to formjoints that have higher fracture-toughness values those of the ceramicsto be joined (see Table 1).

Detail of Ceramic Construction Sans Joint Compound

Surprisingly and unexpectedly, the inventors have found that bodiescomposed of multiple-phase ceramics or other brittle materials can beseamlessly joined without use of a joint compound. The salient featuresof the process are the inverse relation between stress and temperature(lower stresses can be used to form the joint if the temperature isincreased) and the direct relation between temperature and grain size(smaller grain sizes allow for use of lower temperatures). Selection ofparticulate composites, in which no one phase is more than 85 vol. % ofthe total, allows for formation of pore-free joints. The grains of thepieces to be joined must be less than or equal to 10 microns in averagesize. More preferably, they must be less than 3 microns.

The surface finish, as defined by a standard root-mean-square figure ofmerit, must be approximately no greater than 2 microns, and preferablyno greater than 1 micron. In addition, the initial pieces must bepore-free in order for pore-free joints to be fabricated.

FIG. 4 details residual stresses measured by indentation techniques. Theopen symbols represents stresses parallel to the interface while closedsymbols represent stresses perpendicular to the interface.

Detail of Cermet Joining

Joint production in cermet shape processing has some peculiarities. Aswith ceramic shape production, the phases in the cermets also must bekinetically stable. Thin and strong metallic joints in cermets can becreated by plastic deformation. Although the temperature and stressranges used to form the cermet joints are similar to those used for theceramic joints, the cermet joints differ significantly from those inceramics and other hard materials.

In contrast to the invented protocol utilized for ceramic shapeprocessing, the constituents of joint compounds used in joining cermetsare dissimilar to the constituents of the cermets being joined. First,unlike the joining of ceramics, joint compound is usually required incermet joining.

Second, the resulting joint can be multiphase or single phase (inceramics, the joint should be multiphase). Third, an additionalprocessing step is required in which the joint compound is reduced ordried to a metal. Fourth, the resulting joints are stronger than thecermets being joined, but the strength arises from a difference inphysical properties (in this case, inherent ductility because ofactivation of dislocation motion), not from a difference in grain size.

The inventors fabricated joints in low-metal-fraction cermets. A jointcompound is required. In one protocol, flat surfaces having a finish, asdefined by a standard root-mean-square analysis, of 1 micron or less arejoined. Finishes of up to 2 microns are suitable. The surfaces arecleaned by standard techniques, such as washing with a solvent such asacetone or methyl alcohol. In one scenario, the surfaces to be joinedare coated with a solution of metal nitrates or acetates. The solutionmay also contain dispersed, fine particles of carbides or nitrides, suchas WC, TiC, or TiN. These hard particles are generally the same as thosethat are present in the host cermets. The hard carbide and/or nitrideparticles must be smaller than 2 microns for a pore-free joint to beformed. In addition, the volume fraction of hard particle in the finaljoints can be no greater than 0.5. The solution is applied by sequentialdip-coating, spraying, or painting. The solvent can be water, variousalcohols, or other suitable solvents. While any metal is acceptable, themetal in solution is preferred to be similar to the metal in the cermet(for example, transition metals). Suitable metals include Ti, Mn, Fe,Co, Ni, and Zr. Co is the most common.

Once the metal solution is added to the surface, the solution is thendecomposed. One method of decomposition includes heating thesolution-coated surfaces in a reducing atmosphere at moderatetemperatures. For example, the solution coated surfaces can be placed ina hydrogen-gas laden atmosphere, at temperatures between 150 and 800° C.for a time sufficient for the solution to be converted to metal.

After the solution decomposes from the surface, the surfaces are joinedat 1000° C. and under stresses of between 500 and 45,000 psi. Preferablepressure is between 100 psi and 15,000 psi. The atmosphere should bereducing so that no oxide forms in the joint. The pieces to be joinedshould be flat. The resulting joints are one of two types: (1) Piecesjoined directly by a metal such as Co, in which case the joint thicknessshould be approximately less than 5 microns, (2) Pieces joined with acomposite of metal such as Co and up to 50% of the volume consists ofhard particles such as WC, in which case the joint can be up to 10microns thick.

EXAMPLE 1

Dense Al₂O₃/YSZ pellets, denoted ZTA, are joined without an intermediatelayer. The invented protocol allows for one of the pellets to be from 15to 85% Al₂O₃, with the balance being YSZ, and the other pellet beingfrom 0 to 100% Al₂O₃, with the balance being YSZ. The pellets areprepared by blending starting powders well in the case of particulateZTA composites or simply weighing out powders in the case of monolithicceramics. The powders are then cold-pressed into compacts, which aresintered in air, oxygen, or inert atmosphere at temperatures of1400-1600° C.

Thermal-expansion coefficients of the ceramics and composites weremeasured in a Theta Industries Dilatronic (Port Washington, N.Y.).

Specimens were joined in an Instron universal testing apparatus (Canton,Mass.). Once the two surfaces are mated, the construct is placed into anInstron universal testing apparatus (Canton, Mass.). The construct andInstron compression rams are then heated to a temperature that isapproximately 0.6 that of the melting point of Al₂O₃. In the case of thespecific joint in this example, in which 80 vol. % Al₂O₃/20 vol. % YSZis joined to 20 vol. % Al₂O₃/80 vol. % YSZ, the temperature is 1350° C.Initial strain rates for the application of pressure were 10⁻⁵ persecond and the total strain is less than 10%. (The strain figure refersto total compressive deformation relative to the initial height of theentire construct.) Application of a maximum pressure of 1500 psi forless than 1 hour is sufficient to induce enough plastic deformation bygrain-boundary sliding to form a pore-free joint (FIG. 2).

Joined specimens were sectioned, polished, and examined by scanningelectron microscopy (SEM) in a Hitachi S-4700-II (Tokyo, Japan). Vickersmicrohardness measurements on polished specimens were made with a ModelMV-1 hardness tester (Matsuzawa, Ishikawa, Japan).

Any attempt to fabricate the same joint using more typical diffusionalbonding requires temperatures exceeding 1550 C. Also, elaborate (andtherefore expensive) surface preparation is necessary. In the end, anddespite these more elaborate procedures, an imperfect joint results fromconventional joining techniques.

EXAMPLE 2

If an intermediate joint compound is used, the dense bodies to be joinedcan be, but are not limited to, ZTA composites, or pure Al₂O₃, or pureZrO₂. They are prepared in the same manner as described in Example 1.Once they have been sintered, they are coated with the joint material.

In this example, a dense 60 vol. % Al₂O₃/40 vol. % YSZ body is joined toa dense 40 vol. % Al₂O₃/60 vol. % YSZ body. The intermediate layer is 50vol. % Al₂O₃/50 vol. % YSZ (50ZTA), which is applied by aerosolspraying. No surface preparation is required of the bodies to be joined.The joint material is prepared by blending 2 g of 50ZTA, 5 g of solvent,1.2 g of Rohm & Haas AT-51 binder, 0.15 g of Monsanto S-160 plasticizer,and 0.06 g of Avecia S-9000 dispersant. These constituents are placed ina 25 mL polyethylene jar, along with YSZ grinding media. The jar wasthen sealed and placed on a ball-mill rack. The mixture is milled forapproximately 16 hours.

After milling, the mixture is placed into an aerosol-spray nozzle andapplied to the surface of each ZTA body. Nitrogen gas is used as thepropellant. 20 individual passes are used to apply the joint material.The coated bodies are then heated in air to 600° C. to remove theorganics. The surfaces to be joined are mated, and then the resultingconstruct is placed into an Instron universal testing apparatus (Canton,Mass.). The construct and Instron compression rams are then heated to atemperature that is approximately 0.6 that of the melting point ofAl₂O₃. In the case of the specific joint in this example, thetemperature is 1350° C. Initial strain rates for the application ofpressure are 10⁻⁵ per second and the total strain during joining is lessthan 5%. Application of a maximum pressure of 1500 psi for less than 0.5hour is sufficient to induce enough plastic deformation bygrain-boundary sliding to form a pore-free joint.

EXAMPLE 3

A commercial cermet containing approximately 6 vol. % Co metal, with WCand TiC as the hard particles, is bonded. Flat surfaces of the cermetare coated with a metallic nitrate solution. The solution is prepared bydissolving 1 g of Co(NO₃)₂ into 2 g of isopropyl alcohol. The solutionis applied to a surface of each cermet by painting. Each droplet isallowed to dry before the next was added. Two droplets are added to eachsurface to be joined.

The coated surfaces are then heated in a 0.1% H₂/balance Ar atmosphereto 600° C. to decompose the nitrates to metallic Co. The coated surfacesare then mated and then the resulting construct is placed into anInstron universal testing apparatus (Canton, Mass.). The construct andInstron compression rams are then heated to a temperature of 1000° C.Initial strain rates for the application of pressure are 10⁻⁵ per secondand the total strain is less than 1%. Application of a maximum pressureof 1000 psi for less than 1.5 hour is sufficient to induce enoughplastic deformation in the Co to form a pore-free joint (FIG. 6).

While the invention has been described with reference to details of theillustrated embodiment, these details are not intended to limit thescope of the invention as defined in the appended claims. Othermodifications of the materials presented above are also possible.

1. A method for seamlessly joining together objects made of cermet, themethod comprising: a) selecting opposing surfaces of the objects havingsurface finishes as defined by root-mean-square values of up to 2microns; b) coating the surfaces with a fluid containing a metal; c)decomposing the metal containing fluid so as to leave the metal on thesurfaces; and d) contacting the surfaces to each other for a time and ata temperature and pressure sufficient to form an irreversible bondbetween the objects.
 2. The method as recited in claim 1 wherein themetal is identical to a metal contained in the objects.
 3. The method asrecited in claim 1 wherein the metal is Ti, or Co, or Fe, or Mn, or Zr,or Ti-alloy, or Co-alloy, or Fe-alloy, or Mn-alloy, or combinationsthereof.
 4. The method as recited in claim 2 wherein the fluid is ametal solution selected from the group consisting of metallic nitrates,metallic acetates, metallic hydroxides, metallic alkoxides, colloidalsuspension of metals in solvents, or combinations thereof.
 5. The methodas recited in claim 1 wherein the residue has a thickness of fivemicrons or less.
 6. The method as recited in claim 1 wherein the fluidcontains suspended hard particles, the particles selected from the groupconsisting of WC, TiC, TiN, or combinations thereof.
 7. The method asrecited in claim 6 wherein the suspended particles are less than orequal to 2 microns in diameter.
 8. The method as recited in claim 1wherein the residue has a thickness of less than or equal to 10 microns.9. The method as recited in claim 1 wherein the objects undergoingseamless joining are allowed to deform during said joining process. 10.The method as recited in claim 1 wherein the decomposition step occursin a reducing atmosphere.
 11. The method as recited in claim 1 whereinthe decomposition step occurs in an inert atmosphere.
 12. The method asrecited in claim 10 wherein the atmosphere contains hydrogen gasmaintained at a temperature between 150-800° C.
 13. The method asrecited in claim 10 wherein the atmosphere contains hydrogen gas andargon.
 14. The method as recited in claim 10 wherein the atmosphere ismaintained at 150° C.